[GSoC] BAT vs Pages on PowerPC [Please give a feedback]

[Thomas Dörfler]

Hi,

please note that some PPC derivatives (especially those for embedded
projects) usually have eight DBATs and eight IBATs. This feature must be
enabled (e.g. in a 603e core) with a special bit in one of the HID
registers, but it will put the number of BAT ranges closer to what is
needed in embedded computing.

The advantage of BATs versus pages is definitively, that they usually
get set up ONCE during startup and, due to their flexible size, will not
need to be reloaded. Note that the pages usually only cover 4K of
memory, and the "translation lookaside buffer" (TLB) only maintains the
last 16-64 used pages, so there is always code involed reloading the
required page info into the TLB. IMHO this reduces the realtime
performance of the system.

In short: Supporting both (BATs and pages) in libmm would be a good feature.

wkr,

Thomas.

On 22.06.2013 04:11, Chris Johns wrote:
> Hesham Moustafa wrote:
>> Eight: 4 for data and 4 for instructions. BAT areas can not overlap.
> 
> Then I suggest we support for pages as well. There may be a need for
> more areas in the future and I feel 4 may not be enough.
> 
> Chris
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